1. Field of the Invention
The present invention relates to the field of heating, ventilation and air conditioning, and more particularly, to an apparatus for controlling relative humidity in a controlled and energy efficient manner.
2. Description of the Related Art
Heating, ventilation and air conditioning systems and equipment are commonly used to provide control of temperature and relative humidity for comfort and the control of particles and other indoor pollutants in buildings. Recent changes in codes and industry standards from agencies such as the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) mandate and/or suggest changes that require stricter control of relative humidity and increases in the amount of fresh air that circulates in a building. Because fresh, outside air can contain large amounts of moisture, the difficulty in controlling relative humidity in a building increases as the amount of outside air circulation increases. Reducing relative humidity creates a more comfortable, healthier indoor environment for humans by controlling the growth of mold bacteria and viruses.
In a conventional air conditioning system a refrigerant working fluid moves in a cycle from a compressor through a condenser to an evaporator and return to the compressor. A fan creates a flow of air across the evaporator, in the case of a direct expansion system or a cooling coil in the case of a chilled water system, and cools the air to a temperature near or below the dew point. Typically the air leaving the evaporator is saturated with moisture.
In some air conditioning systems, the air is reheated after being cooled by the evaporator. Air moving through an evaporator typically changes temperature from about 80xc2x0 F. to 55xc2x0 F. or lower. As the air is cooled, it reaches the dew point (approximately 60xc2x0 F.) at which temperature moisture removal (dehumidification) begins. As the temperature of the air is lowered below the dew point, additional moisture is removed. Such cooling normally produces air that is colder than desired for human comfort. However, this degree of cooling is often required to provide the necessary amount of dehumidification. In essence, the evaporator, or cooling coil overcools the air in order to remove the moisture. A reheater then heats the air to a comfortable level for humans thereby lowering the relative humidity of the air. This process is very energy inefficient because excess energy is used to overcool and dehumidify the air and even more energy is used to reheat the cold air.
One approach to reheating the air leaving an evaporator is disclosed in U.S. Pat. No. 4,813,474 to Amaze where electric resistance heaters are used for reheating the air. This approach is very energy intensive. In addition, some jurisdictions limit the use of electric resistance heat for reheating air leaving an evaporator to certain applications. One example of this is the state of Florida.
Other approaches to reheating have been disclosed. For example, pumped run-around loops have been disclosed in U.S. Pat. Nos. 5,337,577, 5,228,302 and 5,181,552 all to Eirmann. A limited amount of reheat energy is provided by the run-around loop unless a supplemental heat source is used. This heat source along with the pump used to circulate the fluid in the run-around loop consume additional energy and reduce the energy efficiency of the system. U.S. Pat. No. 5,329,782 to Hyde discloses a liquid amplification pump connected between the outlet of a condenser and a liquid subcooling coil positioned in the air leaving an evaporator.
Another approach, illustrated in U.S. Pat. No. 5,265,433 to Beckwith, discloses an air conditioning system comprised of a subcool coil coupled to the liquid line leaving a condenser through a supplemental heat pipe heat exchanger loop comprised of a heat exchanger and said subcool coil. In addition, a second source of reheat is disclosed being comprised of a second supplemental heat pipe heat exchanger loop comprised of a heat exchanger coupled to the hot gas discharge line of a compressor and a coil disposed in the air leaving the subcool coil. This system is limited as to the amount of reheat it can provide.
In order to increase the moisture removal of air conditioning systems and provide passive reheat, heat pipe heat exchangers have been used. Such systems are disclosed in U.S. Pat. Nos. 2,093,725 to Hull; U.S. Pat. No. 4,607,498 to Dinh; U.S. Pat. No. 4,971,139 to Khattar and U.S. Pat. No. 5,695,004 to Beckwith. In these references, the heat pipe is used to transfer heat from the return air into the supply air of an air conditioning system using the technology of heat pipe heat exchangers.
Heat pipe exchangers are passive devices that are comprised of an evaporator section associated with a heat source, an adiabatic section through which working fluid vapor and, in some designs, working fluid passes and a condenser section in association with a heat sink. In the evaporator section, the working fluid is in a liquid state. Heat absorbed from the heat source causes the working fluid to change from a liquid to a vapor. This vapor passes through the adiabatic region and changes from a vapor to a liquid in the condenser section as heat is given up to the heat sink. The liquid in the condenser is returned to the evaporator by the force of gravity and/or capillary forces through a wick. The return path is either through the adiabatic section or another return passage.
Along these lines, U.S. Pat. No. 5,651,258 to Harris disclose an air conditioning system wherein a coil, in a mode of uncontrolled and continuous operation, is coupled to the liquid line leaving a condenser and disposed in the air leaving an evaporator. The liquid refrigerant is cooled prior to entering the evaporator in which process the cooling capacity of the evaporator is increased and the heat removed from the liquid refrigerant reheats the air leaving the evaporator. In addition the system discloses a coil coupled to the hot gas discharge line of a compressor disposed in the air leaving the subcool coil in a configuration which allows only limited control over the amount of reheat provided by said hot gas reheat coil.
In the types of systems referred to above, the dehumidification capacity of the system is increased and the sensible capacity of the system is decreased. This mode of operation is desirable during times of higher latent load and lower total cooling load, but may not be desirable during times of lower latent load and higher total load. At high loads, the maximum sensible cooling capacity of the system is required to cool the hot air to the desired temperature and less dehumidification (latent) capacity is available to remove moisture from the air. Therefore, it is desirable to control the reheat energy of the system.
One approach to providing fresh air ventilation to a building is illustrated in U.S. Pat. No. 5,179,998 to Des Champs which discloses a system comprised of a cooling coil for cooling and dehumidifying fresh air, a first plate type air-to-air heat exchanger which exchanges heat with the incoming fresh air prior to the cooling coil and exhaust air from a building and second plate type air-to-air heat exchanger which exchange heat with the cool, saturated air leaving the cooling coil and exhaust air from a building. This type of system cools the air prior to entering the cooling coil which reduces the energy required for cooling and reheats the air leaving the cooling coil to reduce the relative humidity of the air prior to entering the building. These types of plate heat exchangers imposes a significant resistance to air flow which must be overcome by the use of a fan motor of greater horsepower in order to achieve the proper airflow. In addition, these types of plate heat exchangers require that the exhaust and fresh air streams be adjacent which can limit the application of this type of system where the air streams are not adjacent.
Therefore, it is an object of this invention to overcome the aforementioned inadequacies of the prior art devices and provide an improvement which is a significant contribution to the advancement of the prior art.
Accordingly, it is an object of this invention to provide an air conditioning system including a controllable subcooling heat exchanger for subcooling liquid refrigerant prior to entering an evaporator while simultaneously reheating the air leaving the evaporator.
It is a further object of this invention to provide an air conditioning system including a hot gas heat exchanger with variable control for reheating air in a post evaporator configuration.
It is a further object of this invention to provide an air conditioning system for providing dehumidified air at the proper temperature for sensible cooling in systems having recirculated high proportions of fresh air or 100% fresh air in the inlet air flow.
It is a further object of this invention to provide an air conditioning system for providing conditioned air which operates with high energy efficiency.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the prominent features and applications of the present invention. It will be apparent to those skilled in art that many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.
For the purposes of summarizing this invention, this invention includes an apparatus which includes a housing having an inlet, an outlet, an evaporator for cooling and dehumidifying air, a blower for inducing air flow through the housing, a subcooling heat exchanger for subcooling liquid refrigerant being delivered to an evaporator from a condenser and for heating air downstream from said evaporator in a controlled manner, a hot gas heat exchanger coupled to the hot gas discharge line of a compressor for further heating air downstream of an evaporator in a controlled manner. The apparatus also preferably comprises an air bypass or face and bypass apparatus for controlling the heating of the subcooling heat exchanger and the hot gas heat exchanger.
In accordance with another aspect of this invention, a control valve, located in the refrigerant liquid line, provides a means of controlling the heating capacity of the subcooling heat exchanger and a valve or a series of valves in the hot gas discharge line of the compressor provides a means of heating capacity to the hot gas heat exchanger.
In accordance with another aspect of this invention, this invention includes an apparatus which includes a housing having an inlet and an outlet, an evaporator or cooling coil for cooling and dehumidifying air, said evaporator comprising the evaporator portion of a refrigeration system, said refrigeration system comprises also of a compressor and condenser, a blower for inducing an air flow through said housing and a blower for inducing air flow across the condenser. A heat pipe heat exchanger comprised of an evaporator section disposed in the leaving air of said condenser and the condenser section of the heat pipe heat exchanger disposed between the evaporator or cooling coil and the outlet of said housing for heating the air. The evaporator section and the condenser section of the heat pipe heat exchanger are interconnected with lines for transporting vaporized working fluid from the evaporator section to the condenser and for returning liquid working fluid form the condenser section to the evaporator section.
In accordance with another aspect of this invention, a first heat exchanger of the air-to-air type is used to cool inlet air to said housing in the cooling mode of operation and heat inlet air in the heating mode of operation by exchanging heat with building exhaust or return and exhaust air mix. A second air-to-air heat exchanger is used to reheat air leaving a cooling coil in the cooling mode of operation and heat air leaving the first heat exchanger in the heating mode of operation. The condenser, in the case of a direct expansion system, is disposed either in the exhaust air upstream of the second heat exchanger to heat the air entering the second heat exchanger and provide a greater capacity for reheating the air leaving the evaporator, or in the exhaust air stream downstream of the first heat exchanger to provide cooler air to the condenser and increase the energy efficiency of the system.